Wireless cellular telephony systems enable voice communication between remotely located persons. They have other uses as well; for example, to permit communication with vehicles equipped with a telematics unit that includes an embedded cellular phone. This permits voice communication between the driver and an advisor at a call center, which can be used to provide various vehicle telematics services, as is known in the art. The voice communication capability provided by the wireless carrier system involves the use of a voice channel that uses vocoders on either side of the wireless transmission to compress the speech in an effort to minimize the amount of data that must be transferred wirelessly.
Digital data communication can also be carried out over this voice channel; and this is done using modems at each end of the connection. The modems are designed to establish a data connection over the voice channel by sending modem signaling tones that are recognized by the receiving modem. However, those signaling tones also pass through the vocoder which may attenuate or even completely eliminate the tones, thereby preventing the desired data connection from being established. This problem is further exacerbated by the existence of different types of vocoders that may use different codecs for compression of the received audio input.